1. Field of the Invention
This invention relates to a photosensor used as an photoelectric transducer which serves as an optical input equipment for various image information processing such as a facsimile and an optical character reader.
2. Description of the Prior Art
It has been generally well-known that a photosensor may be used as an photoelectric transducer for processing image information such as in facsimile, digital copying machines, etc.
In recent years, high-sensitivity readers using a long line-sensor have been known. Remarkable progress has been made in development of these devices as well known to the public.
As an example of a photosensor constituting a long line-sensor, there is a planar photoconductive photosensor comprising a pair of electrodes of metal and the like placed opposite to each other so as to form a gap for a light-receiving portion on a photoconductive layer comprising as photoconductive material chalcogenide, Cds, CdSSe, amorphous silicon (which is hereinafter referred to as "a-Si") and the like.
In such a planar photoconductive photosensor, traveling length l of photo-carriers generated by light incident upon the gap region (light-receiving part) between the electrodes can be expressed by the following equation: l=.mu...tau.. E, wherein .mu. is mobility of the carriers, .tau. is the lifetime of the carriers, and E is electric field intensity in the photoconductive layer. Accordingly, an increase of the electric field intensity E results in the increase of photoelectric current Ip because the traveling length l of the photo-carriers also increases. It is not problem to make photo-gain G at least 1, wherein G=l/L.L is the interelectrode distance at the light-receiving portion.
However, the above discussion in a photoconductive photosensor is not valid unless the electrodes are under ohmic contact with the photoconductive layer. Accordingly, there is generally interposed a layer (hereinafter referred to as an ohmic contact layer) to effect an ohmic contact between the electrodes and the photocoductive layer.
Since the photoconductive photosensor can produce large photoelectric current (signal) as described above, it can stably detect light information even if used for sensors with a minute light-receiving portion such as line-sensors and area-sensors.
A-Si, which is recently attracting attention as a photoconductive material for constituting photoconductive layers, is excellent in photosensitivity, stability, safety, etc., and therefore can be used to fabricate a photoconductive photosensor with excellent characteristics.
However, a-Si is known to be changed in its characteristics under irradiation of light due to the so-called Staebler-Wronski effect [Appl. Phys. Vol 30, 292 (1980)]. In particular, it has been reported that the value of photoelectric current, which is one of important characteristics of photosensors, decreases while the photosensor is continuously irradiated with light. Such decrease of photoelectric current occurs much more dramatically in the photoconductive sensors, as compared to the photodiode type sensors. Consequently, it has been extremely difficult to put a photoconductive sensor using a-Si into practical use.
For example, in a conventional photoconductive photosensor shown in a schematic sectional view of FIG. 1, it has been found in the course of our research that the characteristics of the photosensor significantly deteriorate upon irradiation of light.
In FIG. 1, 11 is a substrate, 12 is a photoconductive layer, 13 is an ohmic contact layer, and 14 are electrodes. That is, this conventional photosensor has the ohmic contact layer interposed between the electrodes and photoconductive layer formed on the substrate.
However, a conventional photosensor with this constitution has a problem in that attempts to prevent the deterioration of the characteristics by light irradiation have been attended by a reduction of light-response characteristics and decrease of photoelectric current.